A blazing oil rig has sunk into the Gulf of Mexico, sparking fears of an environmental disaster two days after a massive blast that left 11 workers missing.
US maritime authorities said crude oil was pouring into the sea at the site where the hulking Deepwater Horizon rig once stood, though they were unable to determine at what rate.
In Washington, President Barack Obama said the federal response to the disaster "was being treated as the number one priority," the White House said in a statement.
Obama "made sure that the entire federal government was offering all assistance needed in the rescue effort as well as in mitigating and responding to the environmental impact," the statement read.
Officials said that before the explosion there were 700,000 gallons (2.6 million liters) of diesel fuel on board the semi-submersible platform and it had been drilling 8,000 barrels, or 336,000 gallons, of oil a day.
"This is considered a major oil spill," Mike O'Berry, a US Coast Guard senior chief petty officer, told AFP.
The Coast Guard said a one mile by five mile slick had settled on the surface some 45 miles (70 kilometers) offshore as a massive clean-up operation gets under way to prevent the oil from hitting land in the Gulf states of Louisiana, Alabama and Mississippi.
Coast Guard vessels continued their search late Thursday for the 11 missing workers, while British oil giant BP, which leased the platform, dispatched a fleet of boats to try to keep environmental damage in check.
Officials said the current spill had the potential to be the worst seen in the United States since the 1989 Exxon Valdez oil tanker spill, considered one of the worst man-made environmental disasters.
That spill, vastly bigger than the current one in the US Gulf, poured nearly 11 million gallons of crude into Alaska's Prince William Sound, devastating some 750 miles (1,300 kilometers) of its once pristine shores.
If oil leaks and the spill escapes containment efforts, Louisiana's sensitive coast would be at risk for ecological damage. Wild birds, breeding grounds for shrimp, and oyster beds would also be threatened. Should a spill reach shore, birds could be covered in the slick.
O'Berry said US authorities were deploying several oil-skimming vessels to try to limit the pollution.
The Coast Guard sent a mini-submarine equipped with cameras to determine the oil flow into the ecologically fragile Gulf, home to a vast array of waterfowl and other wildlife.
Before the rig sank into the ocean, oil fires raged for more than a day and a half following a spectacular explosion late Tuesday that sent huge balls of flame leaping into the night sky.
The now submerged oil rig measuring 396-by-256 feet (121-by-78 meters) is owned by Houston, Texas-based contractor Transocean, Ltd. and under contract to BP.
Transocean confirmed late Thursday that it had not been possible "to stem the flow of hydrocarbons prior to the rig sinking," raising fears that thousands more gallons of crude will pollute the Gulf waters before the flow is contained.
"We are working closely with BP Exploration & Production, Inc. and the US Coast Guard to determine the impact from the sinking of the rig and the plans going forward," the company said.
It added that "the US Coast Guard has plans in place to mitigate any environmental impact from this situation."
Transocean vice president Adrian Rose said earlier that the rig likely suffered a blowout while drilling through rock at BP's Macondo prospect, although investigations into the exact cause of the accident are ongoing.
Meanwhile rescue crews scoured the Gulf waters for sign of the 11 missing workers.
Seventeen workers were airlifted to hospital on Wednesday after suffering broken bones, burns and smoke inhalation in the explosion on the mobile rig. Four remained in critical condition.
But there was no sign of the missing workers and it remained unclear whether they made it safely into one of the rig's lifeboats.
"We're going to continue to search as long as there is a reasonable probability of finding them alive," Coast Guard spokeswoman Ashley Butler told AFP, adding that weather conditions were favorable for the search.
Transocean's Rose said the missing workers "may have been unable to evacuate" once the rig exploded.
Transocean is the world's largest offshore drilling contractor, with a fleet of 140 mobile offshore drilling units, in addition to three ultra-deepwater units under construction.
A total of 126 people were onboard the platform at the time of the explosion, 79 of them Transocean staff, six BP personnel and 41 contractors.
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